This invention relates to a composite of metal and resin and a process for producing the same. More particularly, it relates to a composite of metal and resin which is suitable to be used as a substrate for a printed circuit board and a process for producing the same.
Regarding the adhesion of metals to resins, various methods have hitherto been studied for surface-treatment of metals in order to enhance the adhesive strength between metals and resins. For example, there is known a method which comprises etching the surface of a metal by mechanical means or by means of an oxidizing agent in an acid solution to roughen the surface, then forming an oxidized film on the metal surface frequently in alkaline solution or, even when in acid solution, by utilizing the increase of pH at the surface of the metal due to the reaction of the metal surface with the solution, and then adhering the metal to a resin through the intermediation of the oxidized film. In the case of copper, for example, the surface of metallic copper is roughened by means of etching using an aqueous solution containing cupric chloride and hydrochloric acid as the acid solution, and then an oxidized copper film is formed on the copper surface by using an alkaline solution containing chlorous acid, phosphoric acid, and sodium hydroxide, and the metal is adhered to a resin through the oxidized film at room temperature, or with heating, or with heating under application of pressure. As to the methods for forming an oxidized film on the metal surface, there may also be mentioned a method to conduct the oxidation treatment by using a solution containing potassium permanganate and sodium hydroxide. The oxidized film may also be formed by ultraviolet irradiation or flame treatment. Further, when iron metal is immersed in phosphoric acid, it is oxidized by phosphoric acid and the pH at the surface of iron is elevated as the result of hydrogen generation, whereby a stable phosphate of iron is formed on the iron surface. As a result, resins come to adhere to metals with a high adhesive strength through these metal oxides or metal salts. However, these metal oxides or metal salts have a defect of having poor resistance to acids. The metal-resin composites are frequently used in an atmosphere where contact with acids may take place. Accordingly, the adhesion in the metal-resin composites is desirably not only mechanically strong but also chemically stable.
As to the method for improving the adhesive property of copper coatings, there are known techniques disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 35497/81 and 177593/82. Both of these disclose a technique wherein copper coating is first oxidized and then the copper oxide is reduced under a high-temperature reducing atmosphere until the luster of pure copper appears, whereby a pure metal surface is obtained. Particularly, the latter publication discloses to resolve the problem of brittleness due to polycrystalline fine particles by excluding these particles. That is to say, it is described that when copper metal is rendered to have a lustrous surface having no fine particle, it can form a laminate of high peeling strength. According to the investigation of the present inventors, however, neither of the techniques disclosed in the above references yet gives a sufficiently high adhesive strength. Further, so far as known to the inventors, there has not yet been proposed any effective technique for treating the metal-resin interface to make it acid resistant.